The active implantable devices associated with the present invention typically include a housing, generally designated as a “generator”, that is electrically and mechanically connected to one or more leads. The leads are equipped with electrodes that are intended to come into contact with the patient's myocardium at those sites where the electrical potentials are detected (collected) and/or the stimulation pulses are delivered (applied). These electrodes can be endocardial electrodes (e.g., electrodes that are placed in a cavity of the myocardium in contact with the wall of the myocardium), epicardial electrodes (e.g., electrodes that are preferably used to define a reference potential, or to apply a shock stimulation pulse), or intravascular electrodes (e.g., electrodes that are introduced into the coronary sinus and advanced to a position that faces the myocardial wall of the left ventricle).
Heretofore, an MRI examination was contraindicated for patients having an implanted cardiac pacemaker or defibrillator. This is for several reasons, including, for example:                heating near the electrodes connecting the generator to the patient's heart;        forces and torques of attraction exerted on the device immersed in high intensity magnetic fields generated by an MRI equipment; and        unpredictable behaviour of the device itself, due to exposure to extreme magnetic fields.        
The problem of heating exists especially in the vicinity of leads equipped with electrodes that are connected to the generator. Indeed leads that are placed in an MRI imaging equipment behave like antennas and couple (collect) the radiofrequency (RF) energy emitted by the MRI imager. The frequency of the RF field is equal to the Larmor frequency of protons, f=42.56×B0, where B0, in Tesla, is the characteristic static induction of the MRI imager. For typical static inductions B0 of 1.5 T and 3 T, the RF frequencies correlatively generated by the MRI imager are approximately 64 MHz and 128 MHz respectively.
Resultantly, the RF-induced currents flow in the conductors of the leads immersed in the RF field causing around the electrodes in contact with the blood to heat the surrounding tissues. Indeed, the heating at the electrodes is proportional to the density of current flowing through them. Hence, the smaller the surface of the electrode, the higher the current density and the greater the heating of the surrounding tissues.
In practice, depending on the configuration of the generator, the leads, and the MRI imaging equipment, the temperature rise typically varies from 8° C. (for carbon electrodes) to 12° C. (for metal electrodes), and sometimes even up to 30° C.
The elevated temperature should not exceed 2° C. as specified in the EN 45502-1 standard and its derivatives. At a temperature increase of 4° C. or more, cell death can occur locally. This has as an immediate effect, among others, to irreversibly alter the characteristics of detection and stimulation of cardiac activity.
It is possible, as described in the U.S. Published Application 2007/0255332 A1, to provide a method for safekeeping a device from MRI in which any stimulus is inhibited, and a protection circuit is provided at the connector housing to isolate the conductors of the generator circuits, and to connect all these conductors to the ground of the generator housing to prevent induced parasitic currents. But this procedure prevents the device from functioning for the duration of an MRI examination. An MRI examination can last several minutes, thus it is highly desirable that the device continues during the MRI examination period to provide seamless operation for detecting potential depolarizations at, and deliver stimulation pulses to, the myocardium. To achieve this, during the MRI examination, the device is switched to a protected mode of operation and disables circuits that are sensitive to high magnetic fields, such as RF telemetry circuits, and switching power supplies.
It is thus not sufficient, in practice, to simply disconnect all conductors of the lead and/or to connect them to ground for the duration of an MRI examination, in order to avoid induction of current flow.
To reduce the induced current flow in the lead conductors, it has been proposed to put a filter opposing the current flow in series with the conductors in the path of the induced currents. The filter may be a simple inductor, however, the attenuation of the induced currents is usually not sufficient. It also has been proposed to insert in the current loop an L-C type resonant circuit, tuned to the RF frequency generated by an MRI imager. But this solution has a drawback of requiring different types of filters depending on the RF characteristic frequencies of the MRI imager (e.g., 64 MHz, 128 MHz) because the RF characteristic frequencies vary from one device to another, as explained above.